1. Field of the Invention
The present invention relates to a multilayered ceramic substrate and a method of producing the same. Particularly, a method of producing a multilayered ceramic substrate comprising wiring conductors provided on both main surfaces thereof, and a multilayered ceramic substrate obtained by the producing method.
2. Description of the Related Art
A multilayered ceramic substrate comprises a plurality of laminated ceramic layers. Such a multilayered ceramic substrate also comprises various forms of wiring conductors. As the wiring conductors, for example, an internal conductor film formed in the multilayered ceramic substrate so as to extend in a planar form along a specified interface between the ceramic layers, a via hole conductor is formed to pass through a specified ceramic layer, or an external conductor film is formed to extend in a planar form on the outer surface of the multilayered ceramic substrate.
The internal conductor film or the via hole is partially used for forming a passive part, for example, such as a capacitor element, an inductor element, or the like, in some cases. The external conductor film is used for mounting other chip-shaped electronic parts, and for mounting the multilayered ceramic substrate on a mother board.
In order to further improve multi-functionality, density and performance of the multilayered ceramic substrate, it is effective to arrange the wiring conductors with a high density on the multilayered ceramic substrate. However, a burning step is necessary for obtaining the multilayered ceramic substrate, and the burning step causes shrinkage due to sintering of ceramic. Such shrinkage non-uniformly occurs over the whole of the multilayered substrate, and thus undesired deformation or distortion occurs in the wiring conductors. Such deformation or distortion of the wiring conductors inhibits an increase in the density of the wiring conductors.
Therefore, in producing the multilayered ceramic substrate, it is proposed to apply a so-called non-shrinkage process which is capable of substantially preventing the occurrence of shrinkage in the direction of a main surface of the multilayered ceramic substrate in the burning step.
A method of producing a multilayered ceramic substrate by the non-shrinkage process, which is interesting to the present invention, is disclosed in, for example, U.S. patent application Ser. No. 2,785,544 or Japanese Examined Patent Publication No. 7-46540.
In this technique, dummy green sheets comprising a high-temperature sintered ceramic material such as alumina or the like, which does not sinter at a sintering temperature of low-temperature sintered ceramic materials, are arranged on the upper and lower main surfaces of a laminated structure comprising a plurality of substrate green sheets containing a low-temperature sintered ceramic material, and conductive paste coated for forming internal wiring conductors in connection with the substrate green sheets, the structure is pressed, and then burned at relatively low temperature, and then the unsintered layers derived from the dummy green sheets are removed.
In the burning step, the high-temperature sintered ceramic material is not sintered, and thus substantially no shrinkage occurs in the dummy green sheets, suppressing shrinkage in the direction of the main surfaces of the laminated structure comprising the substrate green sheets due to the force of constraint exerted by the dummy green sheets. Therefore, the laminated structure substantially shrinks only in the direction of the thickness. Less non-uniform deformation occurs in the laminated structure after burning, and thus undesired deformation or distortion of the internal wiring conductors can be prevented.
However, the above-described technique has the following problems to be solved.
First, the high-temperature sintered ceramic material contained in the dummy green sheets is not sintered in the burning step, but mutual diffusion and reaction more or less occurs between the material contained in the dummy green sheets and the material contained in the substrate green sheets in the interfaces between the dummy green sheets and the substrate green sheets, to cause change and deterioration in characteristics of the resultant multilayered ceramic substrate. Particularly, the occurrence of reaction causes difficulties in removing the unsintered layers derived from the dummy green sheets. Therefore, the material which can be used as each of the dummy green sheets and the substrate green sheets is limited.
Also, a sand blasting method, a brushing method using water or a polishing method is used for removing the unsintered layers derived from the dummy green sheets. Therefore, where an external conductor film is desired to be formed on at least one of the main surfaces of the multilayered ceramic substrate, the external conductor film cannot be formed in the stage of the unburned laminated structure. As a result, the external conductor film must be formed on the laminated structure after the unsintered layers are removed after the burning step, thereby causing the need for another step of forming the external conductor film.
Furthermore, the external conductor film must be formed on the laminated structure after burning, and in a case in which a via hole conductor to be connected to the external conductor film is present, there is thus the possibility of causing a failure in conduction between the external conductor film and the via hole conductor.
In producing the multilayered ceramic substrate, the multilayered ceramic substrate is not handled singly, but is handled in the form of an assembly, which is later cut into a plurality of multilayered ceramic substrates. The assembly has the dimensions of as large as a 10 to 20 cm square, and thus warping or waviness occurs on the whole in some cases. Where the external conductor film is formed by, for example, screen printing, the occurrence of such warping or waviness causes difficulties in obtaining high precision of the pattern and position of the external conductor film.
Furthermore, although the above-described method comprises the non-shrinkage process, shrinkage still occurs to some extent, and thus dimensional precision must have an allowance of about xc2x10.1%. Therefore, with respect to the wiring conductors, consideration must be given to such an allowance, thereby inhibiting an increase in density.
One preferred embodiment of the present invention provides a method of producing a multilayered ceramic substrate comprising: preparing a green composite laminated product comprising a plurality of ceramic green sheets to be laminated, conductive paste for forming internal wiring conductors in association with the ceramic green sheets, and metallic foils arranged to cover both main surfaces of a green laminated structure comprising the plurality of ceramic green sheets and to hold the green laminated structure therebetween in the direction of lamination; burning the green composite laminated product at a temperature lower than the melting point of the metal which constitutes the metallic foils, while suppressing shrinkage by the metallic foils in the direction parallel to the main surfaces; and pattering the metallic foils by etching based on a photolithographic technology after the burning step.
In the above described method, each of the ceramic green sheets may comprise a low-temperature sintered ceramic material which can be sintered at a temperature of about 1000xc2x0 C. or less, the conductive paste may comprise a conductive component containing copper or silver as a main component, and the burning step is preferably performed at a temperature of about 1000xc2x0 C. or less.
In the above described method, each of the metallic foils may comprise a copper foil.
In the above described method, the internal wiring conductors formed by the conductive paste may comprise via hole conductors provided to pass through at least one of ceramic green sheets and contact the metallic foils.
In the above described method, the surfaces of the metallic foils which face the green laminated structure may be roughened.
In the above described method, an organic binder and/or glass paste may be provided between at least one of the metallic foils and the green laminated structure.
Another preferred embodiment of the present invention provides a multilayered ceramic substrate obtained by the above described producing method.
In the above described method, shrinkage of ceramic green sheets in the direction parallel to the main surfaces in the burning step can be suppressed by metallic foils, thereby facilitating an increase in density of internal wiring conductors formed in the multilayered ceramic substrate. In addition, the metallic foils are arranged to cover both main surfaces of a green laminated structure comprising a plurality of ceramic green sheets, and to hold the green laminated structure therebetween in the direction of lamination, thereby permitting the formation of external conductor films along both main surfaces of the multilayered ceramic substrate by using the metallic foils.
Furthermore, the metallic foils are patterned by etching based on the photolithographic technology in order to form the external conductor films, thereby easily permitting the formation of the fine external conductor films and the achievement of a higher density. Unlike the prior art method, there is no need to form the external conductor films after the unsintered layers derived from dummy green sheets are removed after burning, thereby decreasing the number of the steps for forming the external conductor films.
The above described method uses the photolithographic technology for forming the external conductor films from the metallic foils, and thus distortion can be corrected by partial exposure, not full exposure. From this viewpoint, the external conductor films can be made further fine, and the density can be further increased.
The above described method does not employ the shrinkage suppressing effect of dummy green sheets, and thus does not have the problem of causing undesired mutual diffusion and reaction between the dummy green sheets and substrate green sheets. Therefore, the ceramic material used for the substrate green sheets can be selected from a wide range.
In the above described method, a conductive paste is provided for forming internal wiring conductors in the stage of a green composite laminated product, and the conductive paste is brought into contact with the metallic foils at necessary positions. In this state, the burning step is performed to prevent the occurrence of a failure in connection in the internal wiring conductors, and between the internal wiring conductors and the metallic foils.
In the above described method, a ceramic green sheet comprises a low-temperature sintered ceramic material which can be sintered at a temperature of about 1000xc2x0 C. or less so that the conductive component contained in conductive pate containing copper or silver as a main component can be used without causing a problem in the burning step at a temperature of about 1000xc2x0 C. or less, thereby realizing the internal wiring conductors with low resistance. Therefore, the resultant multilayered ceramic substrate has good radio-frequency characteristics.
In the above described method, the use of a copper foil as each of the metallic foils permits the formation of external conductor films having low resistance. Since copper has a little problem of migration, the external conductor films can easily be made fine, and the density thereof can easily be increased.
In the above described method, the surfaces of the metallic foils which face the green laminated structure are roughened, or an organic binder and/or glass paste is coated between each of the metallic foils and the green laminated structure so that the adhesive strength between the metallic foils, i.e., the external conductor films, and the laminated structure in the resultant multilayered ceramic substrate can be increased.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.